Analog-to-digital (A/D) circuits are known to comprise devices capable of converting analog wave forms in to corresponding digital representations. Sigma-delta A/D circuits are one type of A/D circuit known in the art. In a sigma-delta A/D converter, the analog wave form is inputted to an integrating lowpass filter, the output of which serves as input to a one bit quantizer. The output of the one bit quantizer serves not only as the digital output of the sigma-delta A/D converter, but it is also fed back and summed with the analog input to produce a difference signal at the input of the integrating lowpass filter. An error signal is produced as the integrating lowpass filter operates on the difference signal. In essence, the sigma-delta A/D converter attempts to produce digital representations of the input signal such that the error signal is continually minimized. Ideally, the resulting power spectral density of the digital representation will substantially match that of the analog input, with the addition of quantization noise. The spectral shape of the quantization noise is greatly influenced by the frequency characteristics of the integrating lowpass filter.
Typically, the integrating lowpass filter utilizes multiple integration stages to realize effective out-of band attenuation of quantization noise. A cost, however, associated with the use of higher order integrating filters is their inherent instability. When an analog input becomes excessively large, this instability can result in uncontrollable oscillations at the output of the sigma-delta A/D converter, and hence, in the loss of information contained in the analog input. Only when the input has been sufficiently reduced will the oscillations cease and normal operations continue. Of course, this situation can be resolved in several ways. Rather than using multi-stage integrating filters, a single stage (single pole) filter can be used. Single pole integrating filters, referred to as first order filters, guarantee stability, but do not offer sufficient noise suppression performance. In another solution, the analog input could be limited prior to the sigma-delta A/D converter thereby ensuring stability. The distortion resulting from the use of limiting, however, could significantly degrade the digital representation.
In radio communication systems, automatic gain control (AGC) is often used to adjust received signals input to a sigma-delta A/D converter. By continually adjusting the amount of gain applied to the received signals, an AGC with a relatively slow attack time helps prevent the overload condition from occurring. When large, instantaneous interfering signals are present, however, the AGC alone cannot prevent an overload condition from occurring and information in the desired signal is lost. Furthermore, the recovery time of the AGC after an overload condition is typically slowed such that the response of the radio receiver is notably slowed.
A solution to this problem has been put forth in U.S. Pat. No. 5,012,244 issued to Wellard et. al. Wellard discloses a sigma-delta A/D converter utilizing a multi-stage integrating filter with an additional oscillation detect and reset circuit. When an unstable condition in the form of oscillations occurs, the oscillation detect and reset circuit causes switches, disposed across the inputs and outputs of the integrating stages, to be closed such that the information present in the circuit is zeroed, and hence lost. In this way, the circuit can reestablish stability that, upon detection, causes the switches to be opened, thus allowing normal operation to continue. A shortcoming of this approach, however, is that during that period of time in which the switches are closed, the converter is inoperable and does not produce any digital representation of the input signal. Therefore, a need exists for a method that facilitates the stability of A/D converters without the use of analog input level reductions or limiting and furthermore allows continuous operation during periods of instability.